1. Field of the Invention
The present invention is directed generally to monitoring operating parameters in a steam turbine and, more specifically, to the monitoring of the vibration of rotating turbine blades.
2. Description of the Prior Art
Turbine blades, because of their complex design, are subject to vibration at frequencies which correspond to natural frequencies of the blades. Each natural frequency is associated with a particular mode which is a different type of vibration such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, etc. In order to prevent excessive vibration of the blade about its normal position, normal design practice dictates that the blades be constructed such that the frequencies of the lowest modes fall between harmonics of the operating frequency of the turbine. It is the case, however, that damaging nonsynchronous, i.e., non-integral harmonic, vibration may occur at the frequencies corresponding to these modes. Typically, nonsynchronous vibration in a steam turbine may occur as a result of buffeting wherein a low steam flow and a high back pressure cause the random excitation of the turbine blades or it may occur as a result of turbine rotor torsional stresses.
The nonsynchronous vibration may result in physical damage to the steam turbine. When the amplitude of the vibration exceeds a certain level, objectionable stresses are set up in the blade. If the condition is not detected and remedied, the blade may eventually fracture resulting in an extremely costly forced outage of the machinery. Thus, a method for detecting that nonsynchronous vibration is necessary to prevent such damage.
One prior art method for detecting turbine blade vibration is to attach strain gages to the turbine blades. Sensor information is communicated to analyzing equipment outside the machine by means of miniature transmitters affixed to the machine's rotating shaft at various locations. This prior art method suffers from three significant drawbacks. First, the strain gages have a very short life due to erosion caused by steam passing through the turbine blades. Second, each blade requires a strain gage if all blades in a row are to be monitored. This represents a significant expense. Additionally, only a limited number of transmitters and, therefore, sensors can be accommodated inside the machine. Third, the complexity of continuously and reliably supplying power to the strain gages and transmitting the signal reliably from the rotating rotor disk to stationary electronics creates severe difficulties.
To obviate these problems, apparatus exist for detecting turbine blade vibration which utilize permanently installed, non-contacting proximity sensors. One such apparatus is disclosed in U.S. Pat. No. 4,573,358 to Luongo wherein a plurality of sensors spaced about the periphery of the blade row detect vibration of operator selected blades. With this apparatus, one blade can be monitored at any given time. Unfortunately, physical difficulties exist in installing and mounting the plurality of sensors needed by the apparatus within the steam turbine. Additionally, this apparatus does not separate the torsional vibrations of the shaft and nonsynchronous vibrations of individual blades.
Another apparatus utilizing non-contacting proximity sensors is U.S. Pat. No. 4,593,566 to Ellis which utilizes two sensors which provide output signals upon the passage of the turbine blades. The disadvantage of this apparatus lies in its less flexible analog processing circuitry, the requirement of three sensors if individual blade vibrations are to be detected, and the non-differentiation between torsional and individual blade vibrations.
Thus, there is a need for a long-lived monitoring system that is easy to install, monitors all blades simultaneously, separates torsional vibration from the nonsynchronous vibration of individual blades, utilizes digital signal processing circuitry and that will measure nonsynchronous turbine blade vibration at a comparatively small expense. The present invention fills the need for such a nonsynchronous turbine blade vibration monitoring system.